We are studying the molecular genetics of inherited hypokalemia and hypomagnesemia, Bartter's and Gitelman's syndromes. The goal of the study is to identify the gene(s) responsible for the phenotype of Bartter's and Gitelman's syndromes and define any phenotypic abnormalities in the relatives of affected patients. The objective of this protocol is to recruit patients with Bartter's and Gitelman's syndomes, patients with unexplained, persistent hypokalemia and/or hypomagnesemia, and related family members in order to permit such studies. We have enrolled 20 patients for impatient evaluation in the GCRC and have enrolled 90 patients for outpatient laboratory evaluation. These numbers include affected patients as well as their unaffected family members. We continue to recruit additional index cases and their family members.